Skin packaging can be classified as a vacuum forming process for thermoformable polymeric films. The product on a supporting member serves as the mold for the thermoformable film which is formed about the product by means of differential air pressure. The term "vacuum skin packaging" (hereinafter "VSP") as used herein indicates that the product is packaged under vacuum and the space containing the product is evacuated from gases. It is therefore desirable that the film formed around the product and for the support member to each present a barrier to oxygen, air, and other gases detrimental to the shelf or storage life of a product such as a food product.
Skin packaging is described in many references, including French Patent No. 1,258,357, French Patent No. 1,286,018, Australian Patent No. 3,491,504, U.S. Pat. No. RE 30,009, U.S. Pat. No. 3,574,642, U.S. Pat. No. 3,681,092, U.S. Pat. No. 3,713,849, U.S. Pat. No. 4,055,672, and U.S. Pat. No. 5,346,735. U.S. Pat. No. 5,346,735 is incorporated herein by reference. Typically, skin packaging provides just a barrier film that upon removal from the package leaves the product exposed to atmosphere, which is sufficient for applications such as bulk meat cutting and repackaging. It is desirable in other applications to provide a composite packaging film comprising both a permeable (i.e., non-barrier) film and a peelable nonpermeable film so that upon removal of the peelable nonpermeable film the product is still selectively protected by the permeable film. This permits a product such as a fresh red meat to be protected by the permeable film but allow the meat to "bloom" from a purplish color to a customer-preferred reddish color upon exposure to oxygen. Commercially available thermoplastic composite films for vacuum skin packaging having both a peelable nonpermeable film and a permeable film include W.R. Grace & Co.'s LDX-2881, LDX-2986, LDX-3690, and LDX-4098.
It is important that the peelable nonpermeable film have certain properties or characteristics. One is that it should adhere to the permeable layer throughout the manufacturing, fabricating, and packaging processes up until the time it is desired to peel it away. It is then important that it readily separates from the permeable film without weakening or tearing the permeable film or otherwise compromising the package integrity. It is therefore important that the peelable nonpermeable and permeable films have sufficient adherence strength without requiring too high a peel strength for separation. Similarly, the permeable film should have sufficient strength so that it will not tear when subjected to the separation forces during peeling. U.S. Pat. No. 5,346,735 discloses examples of peel strength tests of some composite packaging films. The permeable film component of these composite films typically contains additives such as migratable waxes at the nonpermeable film/permeable film interface to facilitate peelability. Examples of such waxes include behenamide, stearamide, and erucamide, to name but a few. However, the use of these migratable additives in the permeable film has disadvantages.
One disadvantage is that the additive may not uniformly migrate to the interface surface. This can result in nonuniform peel strength and uneven peel forces along the interface, which can lead to tears or holes in the permeable film. This tendency is enhanced at locations where the film has been distended during thermoforming, such as at package/product/film boundaries. The resulting tears or holes allow leakage of product purge or juices from the package. The leaking package must then be repackaged by the merchant, causing inconvenience and increasing costs and overhead. Another disadvantage is that migratable additives add to the cost of the film layer materials.
It is therefore desirable to provide a peelable composite packaging film without these disadvantages.